The present embodiments relate to wireless communications systems and, more particularly, to the evolution of High-Mobility receivers for Orthogonal Frequency Division Multiplexing (OFDM) systems applications such as Digital Video Broadcast-Handheld (DVB-H) transmission.
Wireless communications are prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (CDMA) which includes wideband code division multiple access (WCDMA) cellular communications. In CDMA communications, user equipment (UE) (e.g., a hand held cellular phone, personal digital assistant, or other) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA communications are by way of transmitting symbols from a transmitter to a receiver, and the symbols are modulated using a spreading code which consists of a series of binary pulses. The code runs at a higher rate than the symbol rate and determines the actual transmission bandwidth. WCDMA includes alternative methods of data transfer, one being frequency division duplex (FDD) and another being time division duplex (TDD, where the uplink and downlink channels are asymmetric for FDD and symmetric for TDD. Another wireless standard involves time division multiple access (TDMA) apparatus, which also communicate symbols and are used by way of example in cellular systems. TDMA communications are transmitted as a group of packets in a time period, where the time period is divided into time slots so that multiple receivers may each access meaningful information during a different part of that time period. In other words, in a group of TDMA receivers, each receiver is designated a time slot in the time period, and that time slot repeats for each group of successive packets transmitted to the receiver. Accordingly, each receiver is able to identify the information intended for it by synchronizing to the group of packets and then deciphering the time slot corresponding to the given receiver. Given the preceding, CDMA transmissions are receiver-distinguished in response to codes, while TDMA transmissions are receiver-distinguished in response to time slots.
The wireless medium affects a transmitted signal in many ways. For example, the transmitted signal is likely reflected by objects such as the ground, mountains, buildings, and other things that it contacts. Consequently, a received signal is a superposition of variously delayed copies of the transmitted signal. Each such copy is said to be a channel path. The result of these combining paths or channel distortion is that the frequency domain spectrum of the transmitted signal is scaled differently on different frequencies. In addition to channel distortion, the received signal is corrupted by thermal noise in the receiver front end and by other interfering signals. A major task of the receiver is to estimate the transmitted data from the noisy, distorted received signal. This typically requires a channel estimate of channel distortion. Next, equalization uses the channel estimate to correct the channel distortion.
Various approaches have been developed in an effort to reduce or remove the channel effect from the received signal so that the originally-transmitted data is properly recognized. In other words, these approaches endeavor to improve signal-to-noise ratio (SNR), thereby improving other data accuracy measures (e.g., bit error rate (BER), frame error rate (FER), and symbol error rate (SER)). For example, CDMA systems use a spreading code having correlation properties that enable the receiver to detect path delays and scale factors. OFDM systems combat distortion by sending the data in the frequency domain, so channel distortion produces independent scaling of each data signal by a different scale value.
While the preceding approaches provide steady improvements in wireless communications, the present inventors recognize that still further improvements may be made, including by addressing some of the drawbacks of the prior art. In particular, these improvements are directed to improvements in high mobility OFDM receivers in single frequency wireless networks. These improvements may be included in user equipment together with previously described communication modes. Examples of these improvements addressed by embodiments of the present invention include improved FFT window placement and improved channel length estimation. Accordingly, the preferred embodiments described below are directed toward these benefits as well as improving upon the prior art.